Laser induced breakdown spectroscopy (LIBS) is a well-known analytical technique that involves producing a plasma at a surface of a material and analyzing a spectrum of emitted light from the plasma. LIBS provides rapid, in situ, compositional analysis without touching the surface. LIBS is now employed in a wide range of applications such as, the monitoring of active agents in pharmaceutical pills, the detection at a distance of explosives, the determination of the composition of molten metallic alloys and the determination of materials used in ancient paintings and sculptures.
FIG. 1a is a schematic illustration of a typical embodiment of a LIBS system known in the art. The LIBS system includes a short pulse laser, such as Q-switched Nd-YAG laser, that produces plasma at the surface or within the sample (which may be a solid, liquid or gas, or even a complex mixture of any of those such as a slurry). During LIBS, chemical bonds are essentially broken and the material is dissociated into its elemental constituents that are excited to higher level energy states and emit light while losing their excitation energy into lower levels. As each element has its own energy state structure, the wavelengths of the emitted light are used as an index of the elements present in the plasma. Light from the plasma is then collected by a spectrometer, with attendant optical collection system. The intensity of the spectral lines is associated with quantitative information of the elements present in the material.
A typical spectrometer has a light dispersing element, such as a diffractive grating, and the spectrum is recorded by a spectra recording device, such as an array of photomultipliers at defined locations along the dispersed spectrum, or by a camera. The data is then received by a processor.
As with any metrological system, a LIBS system has to be calibrated. This is usually performed with a set of samples of known composition. Since the actually recorded signal depends on many variables such as laser power, ablated mass, atomized mass, plasma temperature, plasma expansion, collected light efficiency, spectrometer dispersing specifications, detector sensitivity, sample characteristics, etc., each recorded spectral signal has to be normalized in order to derive a reproducible measurement.
There are several ways to perform this normalization as explained in the review publication “A review of normalisation techniques in analytical atomic spectrometry with laser sampling: from single to multivariate correction” by N. B. Zorov, A. A. Gorbatenko, T. A. Laburtin, A. M. Popov published in Spectrochimica Acta Part B, vol. 65, pp. 642-657 (2010), but the most widely applied is based on an internal standard. This involves taking a ratio of the line intensity of the analyte (element whose concentration has to be determined) to the one of a reference element (the internal standard) which is present in the analyzed material and in the plasma.
This procedure is shown in FIG. 1b (left side). A spectrum is recorded for a calibration mixture, which has known elemental composition, at least in respect of a reference element and the analyte. An identifying spectral line of the analyte and one of the reference element are then selected. Both lines should be well separated from each other, and from adjacent spectral lines of other expected elements in the sample and calibration mixture, to avoid interference from neighboring emission lines of other elements. Line intensity of the analyte should be proportional to its concentration. Attributes of these lines, either their peak amplitudes or their integrated intensities through the resolving bandwidth of the spectrometer, are then measured and their ratios calculated. By performing measurements on one or more calibration mixtures (internal standards) and of the reference element, a calibration curve may be produced that relates the calculated ratio to the concentration of the analyte. From this calibration curve, knowing the concentration of the reference element, the concentration of the analyte can then be determined.
The right hand side of FIG. 1b shows LIBS assaying according to prior knowledge. The recorded spectrum is received from the LIBS apparatus. Signals from the selected lines of the analyte and reference element, the latter of which having a known concentration in the sample, are measured. A ratio of the signal of the analyte to the signal of the reference element is computed. The calibration curve is used to return a concentration of the analyte.
An implementation of this procedure is illustrated with the example of an aluminum alloy, the reference element being aluminum and the analyte iron. Typically in aluminum alloys, aluminum has a concentration that ranges from 90 to 100%, so a variation of the concentration of a minor element (which is the analyte) does not change significantly the concentration of the reference element. FIG. 2 shows a part of the spectrum between 365 and 407.5 nm of an acquisition spectral window recorded between 250 and 420 nm for 4 samples labeled 5, 6, 10 and 12 and including the 373.49 nm line of neutral iron (Fe I) used for iron concentration determination. The 305.47 nm line of aluminum that is used for the reference element is outside of the shown part in FIG. 2.
FIG. 3 shows a calibration straight line that relates intensity ratio to iron concentration. Such a calibration curve can be used as described above to determine a concentration of iron in an aluminum sample by the LIBS technique.
Unfortunately, this procedure requires a known concentration of a reference element in the sample, and a calibration curve (based on internal standards), to generate the calibration curve. It is often desired to determine composition of complex mixtures of materials in which there is no single major element known beforehand.
It is an object of this invention to provide a method that allows for the determination of the concentration of an analyte in a material for which a major component of the material is not initially known. This method may be implemented in a processor coupled to a LIBS system.
As prior art, the EP 0392337 to Carlhoff et al. describes a method for determining the concentration ratio of two elements (a and b) of an unknown substance from the intensity ratio of two spectral lines of these elements in a plasma of this substance. In accordance with their method, in addition to the intensity ratio, the intensity ratio of two spectral lines 1 and 2 of a third element (c, which may be the same as one of a or b), present in the substance, at different excitation energies E1 and E2 is determined and then the concentration ratio of a to b is determined according to conventional calibration using comparable samples.